This invention is directed to a method of controlling image resolution of a substrate such as a printing plate. The invention provides a method of preparing a printing plate in which a fluid composition is applied by inkjetting the fluid composition onto a substrate to form an oleophilic image area. This invention is also directed to such a printing plate for printing. This invention is further directed to a method of imaging a plate and a method of printing which advantageously minimizes the xe2x80x9cdot spreadingxe2x80x9d of the fluid on a printing plate substrate, thereby avoiding the attendant low resolution and reduced image quality associated with such spreading.
The offset lithographic printing process utilizes a planographic printing plate having oleophilic image areas and hydrophilic non-image areas. The plate is commonly dampened before or during inking with an oil-based ink composition. The dampening process utilizes an aqueous fountain solution such as those described in U.S. Pat. Nos. 3,877,372, 4,278,467 and 4,854,969. When water is applied to the plate, the water will form a film on the hydrophilic areas (i.e. the non-image areas of the plate) but will contract into tiny droplets on the oleophilic plate areas (i.e. the image areas). When a roller carrying an oil-based ink composition is passed over the dampened plate, it will be unable to ink the areas covered by the aqueous film (the non-image areas), but will emulsify the water droplets on the water repellant areas (the image areas) which will then take up ink. The resulting ink image is then typically transferred (xe2x80x9coffsetxe2x80x9d) onto a rubber blanket, which is then used to print onto a medium such as paper.
It has also been proposed to apply xe2x80x9cdirectxe2x80x9d ink jet printing techniques to lithographic printing. For example, European Patent Publication No. 503,621 discloses a direct lithographic plate making method which includes jetting a photocuring ink onto the plate substrate, and exposing the imaged plate substrate to ultraviolet (UV) radiation to harden the image area. An oil-based ink may then be transferred to the image area for printing onto a printing medium. There is no disclosure of the resolution of ink drops jetted onto the substrate, or the durability of the lithographic printing plate with respect to printing runlength.
It has also been proposed to apply the direct ink jet printing techniques without the additional steps of chemical development. This xe2x80x9cnon-processxe2x80x9d approach advantageously results in lower production costs and a more environmentally acceptable printing process. However, in such non-process techniques it is difficult to control the spreading of the fluid which forms the oleophilic ink-accepting portion on the printing plate substrate. Such xe2x80x9cdot spreadingxe2x80x9d causes low printing image resolution and reduced image quality. For example, European Patent Application No. 591,916 A2 discloses a water-based ink having a polymer containing anhydride groups which are thermally cross-linked with a hydroxy-functional polymer. This formulation is applied by jetting the formulation which is at room temperature onto a room temperature substrate. However, this formulation does not achieve good control of dot spreading.
U.S. Pat. No. 4,833,486 discloses the apparatus and process for imaging a plate substrate with a xe2x80x9chot meltxe2x80x9d type of ink jet printer. The image is produced by jetting at high temperature a xe2x80x9cphase changexe2x80x9d type of ink which solidifies when it contacts the cooler substrate. The ink becomes instantaneously solid rather than remaining a liquid or gel which is thereafter cured to form a solid. However, such an ink does not provide good resistance to press run due to the wax-type nature of the ink formulation.
U.S. Pat. No. 5,738,013 discloses a xe2x80x9cmedia/fluidxe2x80x9d system used in the manufacture of lithographic plates. The media is a conventional hydrophilic plate substrate, and the fluid is based on a transition metal complex reactive component. The control of dot spreading via the viscosity differences of the fluid as a function of temperature is not addressed.
European Patent Application No. 776,763 discloses lithographic printing plates having cationic curing epoxy systems which are jetted at high temperature onto a substrate. The viscosity of the heated epoxy curing system is 10 cps at jetting temperature, whereas the viscosity of the system at 25xc2x0 C. is 75 cps.
U.S. Pat. No. 5,511,477 discloses a method for producing a photo polymeric relief printing plate using UV radiation curable ink that may optionally be preheated. Although letter-press, dry-offset, gravure and flexographic printing are disclosed, conventional offset printing is not addressed. The use of the viscosity-temperature relationship of the ink composition to control dot spreading is also not addressed.
Compositions intended for ink jet printers must be carefully formulated to satisfy demanding requirements, which include resistance to drying on the ink jet nozzles while stored in an ink jet print head, drying time, ink density and droplet spreading (expansion) once applied onto the printing substrate. These requirements can conflict. For example, the composition must not clog the small diameter nozzles of the ink jet printer over time. However, the ink must also be capable of rapid drying once it is applied to the printing substrate. In this regard, the drying time for a commercially available ink jet composition on a paper substrate is less than one second, with an advertised resolution of 1,440 dots-per-inch. See generally xe2x80x9cInk-jet Inksxe2x80x9d, The Printing Ink Manual (5th ed. 1993). It is also known to improve the resolution of ink jet printers by applying an ink receiving layer to printable substrates such as metal, plastic, rubber, fabrics, leather, glass and ceramics, prior to printing thereon.
In view of the foregoing, it is an object of this invention to provide a method of controlling spreading of a fluid composition on a substrate. In this regard it would be advantageous to provide a method of preparing a printing plate, particularly for non-process lithographic printing, which provides control of unwanted dot spreading and additionally provides good press run. It is one object of this invention to provide a method, wherein a fluid composition is applied by an ink jet printing apparatus to directly image a plate substrate, and thereby producing a printing plate for lithographic printing. The fluid composition applied to the plate substrate is cured and becomes an ink receptive layer for taking up ink when employed in a lithographic printing process.
It is yet another object of this invention to provide a method of controlling dot spreading for non-process lithographic printing plates.
Additional objects of this invention are to provide a method of imaging a lithographic plate and a method of lithographic printing using such a plate.
The present invention provides for controlling spreading of a fluid on a substrate. The printing plate of this invention is useful in printing processes, particularly so-called xe2x80x9cnon-processxe2x80x9d printing which avoids chemical development steps.
The printing plate is prepared by ink jetting onto a plate substrate a fluid composition having a first viscosity at the temperature of the plate substrate, and having a second viscosity at a second temperature which is the temperature of the fluid composition as it leaves the ink jet head, wherein the ratio of the first viscosity to the second viscosity is about or greater than 300.
In a preferred embodiment, the printing plate is prepared by ink jetting onto a plate substrate a fluid composition having a first viscosity at or about room temperature and having a second viscosity at the temperature of the fluid leaving the ink jet head, typically greater than 25xc2x0 C., wherein the ratio of the first viscosity to the second is about or greater than 300.
In a preferred embodiment the fluid composition comprises at least one noncycloaliphatic epoxy composition, at least one cycloaliphatic epoxy compound, and at least one ultraviolet radiation initiator compound.
This invention is also directed to a method of preparing a printing plate, by controlling the spreading of a fluid composition used to prepare the printing plate by ink jet, thereby avoiding unwanted dot spreading of the fluid composition and an accompanying reduction in image resolution,
This invention is also directed to a method of lithographic printing employing such a plate.